Control system for an exhaust gas sensor

ABSTRACT

An oxygen sensor is mounted in an exhaust path of an internal combustion engine. The status of an exhaust gas is detected in accordance with the output of the oxygen sensor. The oxygen sensor incorporates a heater for heating an element of the sensor. In a region in which the temperature of the sensor element is below 300° C., an adsorbable species becomes adsorbed. In a region in which the exhaust pipe temperature is above 80° C., the adsorbable species becomes adsorbed remarkably. Power supply control is continuously exercised over the heater so as to maintain the sensor element at a temperature of 300° C. or higher until the exhaust pipe temperature drops below 80° C. after an internal combustion engine stop. The power supply to the heater is shut off after the exhaust pipe temperature drops below 80° C.

This is a Divisional of application Ser. No. 10/574,925 filed Apr. 7,2006, which is a National Phase of Application No. PCT/JP2004/018769filed Dec. 9, 2004, which claims the benefit of Japanese PatentApplication No. 2004-015759 filed Jan. 23, 2004. The disclosures of theprior applications are hereby incorporated by reference herein in theirentirety.

BACKGROUND

1. Technical Field

The present invention relates to an exhaust sensor control system, andmore particularly to an exhaust sensor control system for controllingthe status of an exhaust sensor that is mounted in an exhaust path todetect the status of an internal combustion engine exhaust gas.

2. Background Art

In a conventionally known system disclosed, for instance, by JapanesePatent Laid-open No. Hei 4-359142, an oxygen sensor is mounted in aninternal combustion engine exhaust path to detect the status of anexhaust gas. The oxygen sensor generates an output in accordance withthe oxygen concentration in the exhaust gas after reaching its activitytemperature. To this end, the oxygen sensor incorporates a heater, andheated to its activity temperature by the heater while the internalcombustion engine is operating.

The exhaust gas contains a large amount of water vapor. Therefore, ifthe oxygen sensor temperature suddenly drops after an internalcombustion engine stop, a large amount of water is adsorbed by a sensorelement of the oxygen sensor. This water adsorption may give a thermalshock to the sensor element, thereby damaging the sensor element.Therefore, the above-mentioned conventional system continuously heatsthe oxygen sensor for a period of approximately 5 seconds after aninternal combustion engine stop.

When the oxygen sensor is continuously heated for a period ofapproximately 5 seconds after an internal combustion engine stop, thesensor temperature does not suddenly drop. As a result, the proportionof water remaining in the exhaust path that is adsorbed by the sensorelement significantly decreases. As such being the case, theabove-mentioned conventional system can improve the durability of theoxygen sensor by restraining the adsorption of water by the oxygensensor at the time of an internal combustion engine stop.

Including the above-mentioned document, the applicant is aware of thefollowing documents as a related art of the present invention.

[Patent Document 1]

Japanese Patent Laid-open No. Hei 4-359142

[Patent Document 2]

Japanese Patent Laid-open No. Hei 8-75695

[Patent Document 3]

Japanese Utility model Laid-open No. Hei 6-58359

[Patent Document 4]

Japanese Patent Laid-open No. Hei 1-257739

After internal combustion engine startup, the internal combustion engineexhaust sensor is generally heated to a predetermined activitytemperature. In such a heating process, the output from the exhaustsensor temporarily deviates from normal due to the influence ofadsorbable species on the sensor element. It is assumed that theadsorbable species becomes chemically adsorbed to the sensor elementwhen the exhaust sensor temperature lowers after an internal combustionengine stop. The deviation of the exhaust sensor output due to theinfluence of the adsorbable species increases with an increase in theamount of adsorbable species adsorption.

The applicant of the present invention has found that the amount ofadsorbable species adsorption to the exhaust sensor greatly depends onthe exhaust sensor temperature and exhaust gas temperature at theexhaust sensor prevailing after an internal combustion engine stop. Morespecifically, it is found that if the exhaust sensor temperature lowersto reach a temperature region in which the adsorbable species may becomechemically adsorbed (hereinafter referred to as the “absorptiontemperature region”) before the exhaust path temperature sufficientlylowers, a large amount of adsorbable species readily becomes adsorbed.

According to the system disclosed by Japanese Patent Laid-open No. Hei4-359142, the oxygen sensor is continuously heated for a period ofapproximately 5 seconds after an internal combustion engine stop. Thatis, the system has a function for delaying the temperature of theexhaust sensor (oxygen sensor) lowering into the adsorption temperatureregion while the process for lowering the exhaust path temperature is inprogress.

However, the above-mentioned conventional system stops to heat theoxygen sensor before the exhaust path temperature sufficiently lowers.More specifically, the exhaust path temperature does not significantlylower during the 5 second in which the above conventional systemcontinuously heats the oxygen sensor. Therefore, the above conventionalsystem cannot decrease the amount of adsorbable species adsorption.Consequently, the above conventional system cannot restrain the exhaustsensor output from deviating from normal under the influence of theadsorbable species.

SUMMARY

The present invention has been made to solve the above problems andprovides an exhaust sensor control system that is capable of properlydetecting the status of an exhaust gas immediately after internalcombustion engine startup while minimizing the influence of exhaustsensor output deviation, which is caused by an adsorbable species.

The above object is achieved by an exhaust sensor control systemaccording to a first aspect of the present invention. The controllercontrols an exhaust sensor mounted in an exhaust path of an internalcombustion engine. The exhaust sensor includes a sensor element forgenerating an output in accordance with the status of an exhaust gas anda heater for heating the sensor element. The exhaust sensor controlsystem includes a heater control unit for continuing power supplycontrol over the heater until the exhaust gas temperature at the exhaustsensor drops below 80° C. after the internal combustion engine isstopped.

In a first aspect of the present invention, power supply control iscontinued over a heater of an exhaust sensor until the exhaust gastemperature at the exhaust sensor drops below 80° C. This makes itpossible to prevent the temperature of an element of the sensor fromlowering to reach the adsorption temperature region. As a result, thepresent invention effectively restrains the exhaust sensor output fromdeviating from normal under the influence of the adsorbable species byreducing the amount of adsorbable species that becomes adsorbed to theexhaust sensor after an internal combustion engine stop.

In a second aspect of the present invention, the exhaust sensoraccording to the first aspect of the present invention may furtherinclude an element temperature acquisition unit for acquiring thetemperature of the sensor element. The heater control unit includes anafter-stop power supply control unit for controlling the heater with apredetermined temperature between 300° C. and 500° C. set as a targettemperature for the sensor element after the internal combustion engineis stopped.

In a second aspect of the present invention, control can be exercisedafter an internal combustion engine stop so that the temperature of thesensor element is maintained within a target temperature range from 300°C. to 500° C. When control is exercised in this manner, the sensorelement temperature can be efficiently maintained above the adsorptiontemperature region without extra power consumption.

In a third aspect of the present invention, the exhaust sensor accordingto the first or second aspect of the present invention may furtherincludes a heater control unit in which a stop moment exhausttemperature estimation unit and a temperature condition determinationunit are provided. The stop moment exhaust temperature estimation unitestimates the exhaust path temperature at a stop moment of the internalcombustion engine. The temperature condition determination unitdetermines whether the exhaust path temperature is below 80° C. based onthe exhaust path temperature at the stop moment and the elapsed timeafter the internal combustion engine is stopped.

In a third aspect of the present invention, it is possible to estimatethe exhaust path temperature at a moment of an internal combustionengine stop, determine the elapsed time after the internal combustionengine stop, and check, in accordance with the estimated exhaust pathtemperature and determined elapsed time, whether the exhaust pathtemperature is below 80° C. As a result, the present invention cancontinue heating the sensor element for an appropriate period of timewithout directly detecting the exhaust path temperature.

The above object is also achieved by an exhaust sensor control systemaccording to a fourth aspect of the present invention. The controllercontrols an exhaust sensor mounted in an exhaust path of an internalcombustion engine. The exhaust sensor includes a sensor element forgenerating an output in accordance with the status of an exhaust gas anda heater for heating the sensor element. The exhaust sensor controlsystem includes a recovery value counting unit for counting the elapsedtime or the cumulative intake air amount after internal combustionengine startup as a characteristics recovery value. A heater controlunit is provided for controlling the heater with a recovery targettemperature, which is higher than a normal target temperature, set as atarget temperature for the sensor element until the characteristicsrecovery value reaches a recovery determination value. A cumulative leantime counting unit is also provided for counting, after internalcombustion engine startup, the cumulative length of time during whichthe air-fuel ratio is lean. Further, a determination value correctionunit is provided for increasing the characteristics recovery value ordecreasing the recovery determination value with an increase in thecumulative length of time.

In a fourth aspect of the present invention, the early desorption of theadsorbable species can be promoted by controlling the sensor element tomaintain it at a high temperature until the elapsed time or thecumulative intake air amount after internal combustion engine startup(characteristics recovery value) reaches a recovery determination value.As a result, the present invention can eliminate the exhaust sensoroutput deviation, which is caused by the adsorbable species, immediatelyafter internal combustion engine startup. Further, the present inventionensures that the longer the period of time subsequent to internalcombustion engine startup during which the air-fuel ratio is lean, theearlier the characteristics recovery value reaches the recoverydetermination value. In other words, it is possible to reduce the lengthof time during which control is exercised to maintain the sensor elementat a high temperature. When the air-fuel ratio is lean, adsorbablespecies desorption is promoted. Therefore, the longer the period of timeduring which the air-fuel ratio is lean, the shorter the time requiredfor adsorbable species desorption. The present invention makes itpossible to minimize the time during which the sensor element ismaintained at a high temperature in accordance with the time requiredfor adsorbable species desorption.

The above object is also achieved by an exhaust sensor control systemaccording to a fifth aspect of the present invention. The controllercontrols an exhaust sensor mounted in an exhaust path of an internalcombustion engine. The exhaust sensor includes a sensor element forgenerating an output in accordance with the status of an exhaust gas anda heater for heating the sensor element. The exhaust sensor controlsystem includes a recovery value counting unit for counting the elapsedtime or the cumulative intake air amount after internal combustionengine startup as a characteristics recovery value. A heater controlunit is provided for controlling the heater with a recovery targettemperature, which is higher than a normal target temperature, set as atarget temperature for the sensor element until the characteristicsrecovery value reaches a recovery determination value. A stop periodcounting unit is also provided for counting the stop period during whichthe internal combustion engine is stopped. Further, a determinationvalue correction unit is provided for decreasing the characteristicsrecovery value or increasing the recovery determination value with anincrease in the stop period during which the internal combustion engineis stopped.

In a fifth aspect of the present invention, the early desorption of theadsorbable species can be promoted by exercising control to maintain thesensor element at a high temperature until the recovery determinationvalue is reached by the elapsed time or the cumulative intake air amountafter internal combustion engine startup (characteristics recoveryvalue). As a result, the present invention can eliminate the exhaustsensor output deviation, which is caused by the adsorbable species,immediately after internal combustion engine startup. Further, thepresent invention ensures that the longer the period of time duringwhich the internal combustion engine is stopped, the later thecharacteristics recovery value reaches the recovery determination value.In other words, it is possible to exercise control to maintain thesensor element at a high temperature for an extended period of time. Thelonger the period of time during which the internal combustion engine isstopped, the larger the amount of adsorbable species adsorption.Therefore, the longer the period of time during which the internalcombustion engine is stopped, the longer the time required foradsorbable species desorption. The present invention makes it possibleto minimize the time during which the sensor element is maintained at ahigh temperature in accordance with the time required for adsorbablespecies desorption.

The above object is also achieved by an exhaust sensor control systemaccording to a sixth aspect of the present invention. The controllercontrols an exhaust sensor mounted in an exhaust path of an internalcombustion engine. The exhaust sensor includes a sensor element forgenerating an output in accordance with the status of an exhaust gas anda heater for heating the sensor element. The exhaust sensor controlsystem includes a cumulative lean time counting unit for counting, afterinternal combustion engine startup, the cumulative length of time duringwhich the air-fuel ratio is lean. The exhaust sensor control system alsoincludes a heater control unit for controlling the heater with arecovery target temperature, which is higher than a normal targettemperature, set as a target temperature for the sensor element untilthe cumulative length of time reaches a recovery determination value.

In a sixth aspect of the present invention, the early desorption of theadsorbable species can be promoted by exercising control to maintain thesensor element at a high temperature until the cumulative time duringwhich the air-fuel ratio is lean reaches the recovery determinationvalue after internal combustion engine startup. Since adsorbable speciesdesorption is promoted when the air-fuel ratio is lean, it can beconcluded that adsorbable species desorption is completed when thecumulative lean time reaches the recovery determination value. As aresult, when it is concluded that adsorbable species desorption iscompleted after internal combustion engine startup, the presentinvention can properly terminate a high-temperature control process forthe exhaust sensor.

In a seventh aspect of the present invention, the exhaust sensor controlsystem according to the sixth aspect of the present invention mayfurther includes a recovery value counting unit for counting the elapsedtime or the cumulative intake air amount after internal combustionengine startup as a characteristics recovery value. The controller alsoincludes a determination value correction unit for increasing thecumulative length of time or decreasing the recovery determination valuewith an increase in the characteristics recovery value.

A seventh aspect of the present invention ensures that the time requiredfor the cumulative lean time to reach the recovery determination valuedecreases with an increase in the elapsed time or the cumulative intakeair amount after internal combustion engine startup (characteristicsrecovery value). In other words, it is possible to reduce the period oftime during which control is exercised to maintain the sensor element ata high temperature. Adsorbable species desorption progresses with anincrease in the characteristics recovery value no matter whether theair-fuel ratio is lean. The present invention makes it possible to takethe progress of adsorbable species desorption into consideration andaccurately minimize the period of time during which high-temperaturecontrol is exercised over the exhaust sensor.

In an eighth aspect of the present invention, the exhaust sensoraccording to the sixth or seventh aspect of the present invention mayfurther includes a stop period counting unit for counting the stopperiod during which the internal combustion engine is stopped. Thecontroller also includes a determination value correction unit fordecreasing the cumulative length of time or increasing the recoverydetermination value with an increase in the stop period during which theinternal combustion engine is stopped.

In an eighth aspect of the present invention, the time at which thecumulative lean time reaches to the recovery determination value delayswith an increase in the stop period during which the internal combustionengine is stopped. In other words, the period during which the sensorelement is maintained at a high temperature is enlarged with theincrease of the stop period. Since the amount of adsorbable speciesadsorption increases with an increase in the stop period during whichthe internal combustion engine is stopped, the longer the time duringwhich the internal combustion engine is stopped, the longer the timerequired for adsorbable species desorption. The present invention makesit possible to take the resulting influence into consideration andaccurately minimize the period of time during which high-temperaturecontrol is exercised over the exhaust sensor.

The above object is also achieved by an exhaust sensor control systemaccording to a ninth aspect of the present invention. The controllercontrols an exhaust sensor mounted in an exhaust path of an internalcombustion engine. The exhaust sensor includes a sensor element forgenerating an output in accordance with the status of an exhaust gas anda heater for heating the sensor element. The exhaust sensor controlsystem includes an element temperature acquisition unit for acquiringthe temperature of the sensor element. The controller also includes adesorption progress value counting unit for counting the elapsed time orthe cumulative intake air amount after the temperature of the sensorelement reaches the desorption temperature of an adsorbable speciesadsorbed by the sensor element as a desorption progress value. An outputcorrection unit is provided for correcting the output of the exhaustsensor in accordance with a sensor output correction value. Further, acorrection value calculation unit is provided for decreasing the sensoroutput correction value with an increase in the desorption progressvalue.

In a ninth aspect of the present invention, it is possible to count theelapsed time or the cumulative intake air amount after the temperatureof the sensor element reaches the adsorbable species desorptiontemperature as a desorption progress value. A deviation is superposedover the exhaust sensor output when the adsorbable species becomesdesorbed. The amount of such a deviation decreases with an increase inthe desorption progress value. In the present invention, the exhaustsensor output is corrected by a sensor output correction value. Thesensor output correction value decreases with an increase in thedesorption progress value. As a result, the present invention makes itpossible to accurately compensate for an output deviation, which iscaused by adsorbable species desorption, and obtain a sensor output thatis not affected by the adsorbable species.

In a tenth aspect of the present invention, the exhaust sensor accordingto the ninth aspect of the present invention may further includes a stopperiod counting unit for counting the stop period during which theinternal combustion engine is stopped. The correction value calculationunit includes initial value setup unit, which increases the initialvalue for the sensor output correction value with an increase in thestop period.

A tenth aspect of the present invention ensures that the initial valuefor the sensor output correction value increases with an increase in theperiod during which the internal combustion engine is stopped. If theinternal combustion engine is stopped for a long period of time, anincreased amount of adsorbable species becomes adsorbed; therefore, thesensor output is likely to suffer significant deviation. The presentinvention can accurately correct the sensor output because the amount ofadsorbable species adsorption can be reflected in the initial value forthe sensor output correction value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the structure of an oxygen sensor according to afirst embodiment of the present invention;

FIG. 2 is a block diagram illustrating the configuration of a controlleraccording to the first embodiment of the present invention;

FIG. 3A shows how an adsorbable species is adsorbed by a sensor elementafter an internal combustion engine stop;

FIG. 3B shows how the adsorbable species affects the oxygen sensoroutput after internal combustion engine startup;

FIG. 4 schematically shows the relationship among the temperature of anexhaust pipe, the temperature of a sensor element and the readiness withwhich the adsorbable species adsorbs to the sensor element;

FIG. 5 is a flowchart illustrating a routine that is executed by thefirst embodiment of the present invention;

FIG. 6 shows an example of a map that is referenced by the routine shownin FIG. 5;

FIG. 7 is a flowchart illustrating a modified routine that is executedby the first embodiment of the present invention;

FIG. 8 is a flowchart illustrating a routine that is executed by asecond embodiment of the present invention;

FIG. 9 is a flowchart illustrating a routine that is executed by a thirdembodiment of the present invention;

FIG. 10 is a flowchart illustrating a routine that is executed by afourth embodiment of the present invention;

FIG. 11 is a flowchart illustrating a routine that is executed by afifth embodiment of the present invention;

FIGS. 12A and 12B show examples of a map that is referenced when theroutine shown in FIG. 11 is executed; and

FIG. 12C illustrates how a final correction value calculated by theroutine shown in FIG. 11 changes with time.

DETAILED DESCRIPTION OF EMBODIMENTS First Embodiment HardwareConfiguration of the First Embodiment

FIG. 1 illustrates the configuration of an oxygen sensor 10 that is usedaccording to a first embodiment of the present invention. The oxygensensor 10 shown in FIG. 1 is positioned in an exhaust path of aninternal combustion engine to detect whether oxygen exists in an exhaustgas, that is, determine whether the exhaust air-fuel ratio is lean orrich.

The oxygen sensor 10 is provided with a cover 12. The cover 12 isinstalled over the exhaust path in such a manner that it is exposed tothe exhaust gas. The cover 12 is internally provided with a hole (notshown) for exhaust gas guidance. A sensor element 14 is mounted withinthe cover 12. The sensor element 14 is shaped like a tube, one end ofwhich (lower end in FIG. 1) is closed. The sensor element 14 comprises asolid electrolyte layer 16, an exhaust electrode 18, and an atmosphericair electrode 20. The solid electrolyte layer 16 comprises a ZrO₂ solidelectrolyte. The exhaust electrode 18 and atmospheric air electrode 20,which both consist of a highly catalytic Pt based metal, are formedrespectively on the outer side and inner side of the solid electrolytelayer 16.

An atmospheric air chamber 22, which is exposed to atmosphere, is formedinside the sensor element 14. A heater 24 for heating the sensor element14 is positioned in the atmospheric air chamber 22. When heated to anactivity temperature between approximately 550° C. and 600° C., thesensor element 14 becomes active and ready to generate a steady output.When a control circuit, which will be described later, exercises powersupply control, the heater 24 can heat the sensor element 14 to theabove-mentioned activity temperature.

FIG. 2 is a block diagram illustrating a controller for the oxygensensor 10. A control circuit according to the present embodimentcomprises a sensor control microcomputer 30 (hereinafter abbreviated tomicrocomputer 30) and an engine control ECU (Electronic Control Unit) 40(hereinafter abbreviated to ECU 40). A battery supplies electrical powerto the microcomputer 30. Further, a vehicle's ignition switch (IGswitch) supplies an ignition ON/OFF signal to the microcomputer 30. Themicrocomputer 30 incorporates a timer circuit, which allows themicrocomputer 30 to operate for a predetermined period of time afterignition switch OFF.

The microcomputer 30 is connected to the sensor element 14 of the oxygensensor 10 (strictly, the exhaust electrode 18 and atmospheric airelectrode 20) and the heater 24. The exhaust electrode 18 and theatmospheric air electrode 20 of the sensor element 14 generateelectromotive force therebetween. The electromotive force generated inthis manner varies depending on whether oxygen exists in the exhaustgas. The microcomputer 30 acquires the electromotive force as a sensoroutput and determines whether oxygen exists in the exhaust gas, that is,whether the exhaust air-fuel ratio is lean or rich.

Further, the microcomputer 30 has a function for detecting the impedanceof the sensor element 14 by a publicly known method. Since the impedanceof the sensor element 14 corresponds to the temperature of the sensorelement 14, the microcomputer 30 can estimate the sensor elementtemperature from the impedance. The microcomputer 30 can thereforeexercise feedback control over power supply to the heater 24 so that thesensor element temperature reaches a target level.

The microcomputer and ECU 40 exchange sensor information (the outputfrom the oxygen sensor 10 or the like) and engine/vehicle information(intake air amount, water temperature, etc.). The ECU 40 uses thereceived sensor information, for instance, to provide air-fuel ratiofeedback for the fuel injection amount. The microcomputer 30 uses thereceived vehicle information, for instance, to estimate the exhaust pathtemperature.

[Influence of Adsorbable Species]

FIG. 3A illustrates how the adsorbable species becomes adsorbed to thesensor element 14 after an internal combustion engine stop. FIG. 3Billustrates how the adsorbable species affects the output of the oxygensensor 10 after internal combustion engine startup.

As described earlier, the oxygen sensor 10 is used while the exhaustelectrode 18 is exposed to the exhaust gas. The exhaust gas containsH₂O, CO₂, O₂, and various other constituents. While the oxygen sensor 10is active, such constituents do not become adsorbed to the exhaustelectrode 18. However, a chemical adsorptive reaction may occur betweensuch constituents and the exhaust electrode 18 when the temperature ofthe sensor element 14 lowers after an internal combustion engine stop.

The applicant of the present invention has found that the aboveadsorptive reaction is likely to occur particularly when the temperatureof the sensor element 14 is below 300° C. In other words, the applicantof the present invention has found that the upper limit on thetemperature region in which the adsorbable species becomes adsorbed tothe sensor element 14 (adsorption temperature region) is approximately300° C. Since the power supply to the heater 24 stops after the internalcombustion engine 10 is stopped, the temperature of the sensor element14 inevitably lowers to an adsorption temperature region, which is below300° C. As a result, the adsorbable species inevitably becomes adsorbedto the surface of the sensor element 14 after the internal combustionengine 10 is stopped, as shown in FIG. 3A.

When the internal combustion engine starts up, the power supply to theheater 24 starts. Therefore, the temperature of the sensor element 14rises. When the temperature of the sensor element 14 rises above theadsorption temperature region, that is, exceeds 300° C., the adsorbablespecies begins to become desorbed from the surface of the exhaustelectrode 18 as shown in FIG. 3B and various reactions vigorously occuron the surface. In such an instance, because H₂, which is a reducingsubstance, is generated on the surface of the exhaust electrode 18 andbecause the number of points for reaction with oxygen on the exhaustelectrode 18 decreases due to the presence of the adsorbable species,the output of the oxygen sensor 10 is temporarily shifted toward therich side. When the temperature of the sensor element 14 rises and thedesorption of the adsorbable species progresses, the rich displacementof the sensor output is compensated for in the end.

Solutions Provided by the First Embodiment

To accurately control the air-fuel ratio of the internal combustionengine immediately after it is started up, it is preferred that theoutput of the oxygen sensor 10 be available as soon as possible. Thesensor output cannot be directly used if it is affected by richdisplacement. It is therefore preferred that adsorbable speciesdesorption be completed early. To assure early completion of adsorbablespecies desorption, it is preferred that the amount of adsorbablespecies adsorbed by the sensor element 14 during an internal combustionengine stop be minimized.

FIG. 4 schematically shows the relationship among the temperature of anexhaust pipe, the temperature of a sensor element and the readiness withwhich the adsorbable species adsorbs to the sensor element. As indicatedin FIG. 4, the applicant of the present invention has found that thereadiness with which the adsorbable species becomes adsorbedsignificantly varies with the temperature of the sensor element 14 andthe temperature of an exhaust pipe. That is, the applicant of thepresent invention has found that if the temperature of the sensorelement 14 is above 300° C., adsorbable species adsorption does notoccur without regard to the exhaust pipe temperature. If the elementtemperature is below 300° C. and within the adsorption temperatureregion, adsorbable species adsorption readily occurs while the exhaustpipe temperature is above 80° C., but does not readily occur while theexhaust pipe temperature is below 80° C.

According to the characteristics shown in FIG. 4, adsorbable speciesadsorption does not occur if the sensor element 14 is maintained at atemperature higher than 300° C. during the time interval between theinstant at which the internal combustion engine is stopped and theinstant at which the exhaust pipe temperature drops below 80° C.(condition 1). If the element temperature drops below 300° C. after theexhaust pipe temperature is below 80° C. (condition 2), it is possibleto sufficiently restrain the amount of adsorbable species that isgenerated while the element temperature lowers to a normal level. Underthese circumstances, the present embodiment controls the heater 24 forthe oxygen sensor 10 so that conditions 1 and 2 above are met after theinternal combustion engine's ignition switch is turned OFF.

Process Performed by the First Embodiment

FIG. 5 is a flowchart illustrating a routine that the microcomputer 30executes to implement the above functionality. The routine shown in FIG.5 is started upon internal combustion engine startup and then repeatedlyexecuted at predetermined time intervals.

In the routine shown in FIG. 5, step 100 is first performed to determinewhether it is recognized that the internal combustion engine is stopped.More specifically, the routine checks whether an OFF output is generatedby the ignition switch. If the stop of the internal combustion engine isnot recognized in step 100, step 101 is performed to calculate thecumulative value GAsum of the intake air amount Ga. Next, step 102 isperformed to exercise impedance feedback control over the oxygen sensor10. A normal value (which represents an appropriate temperature withinthe range from 550° C. to 600° C.) is employed as a target temperaturefor the sensor element 14. Therefore, when processing step 102 isperformed, control is exercised to maintain the element at an activitytemperature between 550° C. and 600° C.

If, on the other hand, the stop of the internal combustion engine isrecognized in step 100, step 104 is performed to increment a counterTENGSP that counts the elapsed time after an internal combustion enginestop. It is assumed that the value of counter TENGSP is reset to zerowhen an initialization process is performed upon internal combustionengine startup. For the sake of simplicity, the count reached by counterTENGSP will be referred to as the “elapsed time TENGSP”.

Next, the target temperature for the sensor element 14 changes to 300°C., that is, the lower-limit temperature that is outside the adsorptiontemperature region. More specifically, a target impedance Ztg that isused for impedance feedback control is changed to a value Z300 thatcorresponds to an element temperature of 300° C. (step 106).

The target time interval between the instant at which the internalcombustion engine is stopped and the instant at which the power supplyto the heater 24 is stopped (hereinafter referred to as the “target stoptime THTSP”) is then calculated in accordance with the intake air amountcumulative value GAsum (step 108).

In the present embodiment, the target stop time THTSP should coincidewith the time required for the exhaust pipe temperature to drop below80° C. This required time increases with an increase in the exhaust pipetemperature that prevails while the internal combustion engine isstopped. Meanwhile, the exhaust pipe temperature gradually rises to aconvergence value after internal combustion engine startup. It istherefore believed that the temperature increases with an increase inthe intake air amount cumulative value GAsum. Thus, it is conceivablethat the above-mentioned required time, which should coincide with thetarget stop time THTSP, increases (up to an upper limit) with anincrease in the intake air amount cumulative value GAsum, which iscalculated at the time of an internal combustion engine stop.

FIG. 6 shows an example of a map that the microcomputer 30 references tocalculate the target stop time THTSP in step 108 above. The map is setso that the target stop time THTSP increases with an increase in theintake air amount cumulative value GAsum and coincides with theabove-mentioned required time. According to the process performed instep 108, the time that accurately matches the required time intervalbetween the instant at which the internal combustion engine is stoppedand the instant at which the exhaust pipe temperature drops below 80° C.can be set as the target stop time THTSP.

Next, step 110 is performed to determine whether the elapsed time TENGSPafter an internal combustion engine stop has reached the target stoptime THTSP. If the determination result does not indicate thatTENGSP≧THTSP, it can be estimated that the exhaust pipe temperature isnot yet below 80° C. In this instance, impedance feedback control issubsequently exercised in step 102. Since the target temperature for theelement is set to 300° C. by the process performed in step 104, theheater 24 is controlled so that the sensor element 14 reaches atemperature of 300° C.

For the purpose of controlling the heater 24 to attain a sensor elementtemperature of 300° C., the above process is repeatedly performed untilit is found that TENGSP≧THTSP, that is, until it is estimated that theexhaust pipe temperature is below 80° C. If the temperature of thesensor element 14 does not drop below 300° C., the adsorbable speciesdoes not become adsorbed to the sensor element 14. If control isexercised to maintain the sensor element 14 at a temperature of 300° C.,it is possible to avoid adsorbable species adsorption while the powerconsumption is minimized. As a result, the controller according to thepresent embodiment can efficiently restrain adsorbable speciesadsorption during the time interval between the instant at which theinternal combustion engine is stopped and the instant at which theexhaust pipe temperature drops below 80° C.

When an adequate amount of time elapses after an internal combustionengine stop, the determination result obtained in step 110 indicatesthat TENGSP≧THTSP. In this instance, it can be estimated that theexhaust pipe temperature is below 80° C. When it is found thatTENGSP≧THTSP, the routine shown in FIG. 5 shuts off the power supply tothe heater 24 (sets the heater power supply duty RDUTY to zero) andcauses the microcomputer 30 to terminate its heater control (step 112).

After the power supply to the heater 24 is shut off, the temperature ofthe sensor element 14 decreases from 300° C. to a normal temperature.During such an element temperature drop, the adsorbable species becomesadsorbed to the sensor element 14. However, the exhaust gas temperatureand humidity at the sensor element 14 are no longer high. Therefore, asmall amount of adsorbable species becomes adsorbed. If the amount ofadsorbable species adsorption is small, the influence of the adsorbablespecies immediately disappears after a subsequent internal combustionengine restart. As a result, the controller according to the presentembodiment minimizes the influence of oxygen sensor output deviation,which is caused by the adsorbable species, and begins to properly detectthe exhaust gas status immediately after internal combustion enginestartup.

Example of a Modified Process Performed by the First Embodiment

A modified process performed by the first embodiment will now bedescribed. The adsorbable species, which causes the oxygen sensor outputto suffer rich displacement, becomes chemically adsorbed to the oxygensensor 10. In addition, the carbon content of the exhaust gas may alsobecome adsorbed to the oxygen sensor 10. The carbon content can beburned off by heating the sensor element 14 to a temperature, forinstance, of approximately 700° C.

As described earlier, the controller according to the present embodimentcontinues to control the heater 24 until the exhaust pipe pathtemperature drops below 80° C. after an internal combustion engine stop.If the sensor element 14 is temporarily heated to a temperature of 700°C. in such an instance, the carbon content can be burned off whilereducing the amount of adsorbable species adsorption.

FIG. 7 shows a routine that is executed to implement the abovefunctionality. The routine shown in FIG. 7 is the same as the routineshown in FIG. 5 except that a carbon burn-off process is insertedbetween steps 104 and 106. As regards the steps in FIG. 7 that are thesame as the steps in FIG. 5, their description is omitted or abridgedwith the same reference numerals assigned.

If the determination result obtained in step 100 indicates that theinternal combustion engine is stopped, the routine shown in FIG. 7performs processing step 104 and then changes the target temperature forthe sensor element 14 to 700° C., that is, to a temperature for burningoff the carbon content. More specifically, the target impedance Ztg foruse in impedance feedback control is changed to a value Z700 thatcorresponds to an element temperature of 700° C. (step 120).

Next, step 122 is performed to determine whether the length of timeduring which the sensor element 14 is maintained at 700° C. has exceeded5 seconds. Step 102 is performed to exercise impedance feedback controlwith the target temperature set at 700° C. until it is determined thatthe sensor element 14 has been maintained at 700° C. for a period oflonger than 5 seconds. After it is determined that the sensor element 14has been maintained at 700° C. for a period of longer than 5 seconds,step 106 is performed to change the target temperature for the sensorelement 14 to 300° C.

According to the above processing steps, the sensor element 14 ismaintained at 700° C. for a period of approximately 5 seconds after aninternal combustion engine stop. Subsequently, control is exercised tomaintain the sensor element temperature at 300° C. until the exhaustpipe temperature drops below 80° C. If the sensor element 14 ismaintained at 700° C. for a period of 5 seconds, the carbon attached tothe sensor element 14 burns off. As a result, when the microcomputer 30executes the routine shown in FIG. 7, the carbon content can be burnedoff immediately after an internal combustion engine stop whilerestraining the amount of adsorbable species adsorption to the sensorelement 14 during a temperature drop process.

In the first embodiment, which is described above, the sensor mounted inthe exhaust path is limited to the oxygen sensor 10. Alternatively,however, an air-fuel ratio sensor that generates an output linear withrespect to the exhaust air-fuel ratio may be used in place of the oxygensensor. This also holds true for the other embodiments described later.

In the first embodiment, which is described above, the targettemperature for the sensor element 14 is set to 300° C. for the purposeof avoiding adsorbable species adsorption with a minimum of powerconsumption during the time interval between the instant at which theinternal combustion engine is stopped and the instant at which theexhaust pipe temperature reaches 80° C. However, an alternative targettemperature may be set for the sensor element 14. More specifically, atarget element temperature of 300° C. or higher is acceptable. From theviewpoint of power consumption reduction, it is preferred that thetarget element temperature be between 300° C. and 500° C. or so.

In the first embodiment, which is described above, “heater control unit”according to the first aspect of the present invention is implementedwhen the microcomputer 30 performs processing steps 102 through 110.Further, “element temperature acquisition unit” according to the secondaspect of the present invention is implemented when the microcomputer 30acquires the sensor element impedance, and “after-stop power supplycontrol unit” according to the second aspect of the present invention isimplemented when the microcomputer 30 performs processing steps 106 and102 in order named. Furthermore, “stop period exhaust temperatureestimation unit” and “temperature condition determination unit”according to the third aspect of the present invention are implementedwhen the microcomputer 30 performs processing steps 101, 108, and 110.

Second Embodiment Features of the Second Embodiment

The second embodiment of the present invention will now be describedwith reference to FIG. 8. The controller according to the secondembodiment is implemented when the hardware configuration shown in FIG.1 or 2 is employed to let the microcomputer 30 execute a routine shownin FIG. 8 in place of or together with the routine shown in FIG. 5 or 7.

The controller according to the first embodiment can reduce the amountof adsorbable species that becomes adsorbed to the sensor element 14during an internal combustion engine stop. However, the completeprevention of the adsorbable species adsorption is impossible by thecontroller. The controller according to the first embodiment cannotprevent the output of the oxygen sensor 10 from temporarily sufferingrich displacement after internal combustion engine startup.

The adsorbable species adsorbed by the sensor element 14 begins tobecome desorbed when the temperature of the sensor element 14 exceeds300° C. The higher the temperature of the sensor element 14, the higherthe speed of desorption. Therefore, when the sensor element 14 is set toa temperature (e.g., 800° C.) higher than a normal target temperature(550° C. to 600° C. or so), the time required for adsorbable speciesdesorption can be reduced to shorten the length of time during which thesensor output suffers rich displacement. This control process isthereinafter referred to as “high-temperature control”.

From the viewpoint of sensor element durability and power consumption,it is preferred that the length of time during which control isexercised to maintain the sensor element 14 at a temperature higher thanthe normal target temperature be minimized. Therefore, if suchhigh-temperature control is exercised over the sensor element 14, it ispreferred that such high-temperature control terminate immediately aftercompletion of adsorbable species desorption.

The applicant of the present invention has found that the speed ofadsorbable species desorption depends on not only the temperature of thesensor element 14 but also the air-fuel ratio of the exhaust gassurrounding the sensor element 14. More specifically, the applicant ofthe present invention has found that if the exhaust gas is an oxidativeatmosphere, that is, if the exhaust air-fuel ratio is lean, adsorbablespecies desorption is accelerated because the adsorbable speciesgenerates H₂, which is a reducing substance, at the time of desorption.Therefore, the time required for the completion of adsorbable speciesdesorption decreases with an increase in the “lean time”, whichrepresents the period of time subsequent to internal combustion enginestartup during which the exhaust air-fuel ratio remains lean.

When the above requirements are considered, if the time required foradsorbable species desorption shows increase or decrease, thehigh-temperature control period for the sensor element 14 should bevaried in accordance with the increase or decrease. Under thesecircumstances, the present invention exercises high-temperature controlover the sensor element 14 while counting the cumulative lean time afterinternal combustion engine startup to ensure that the length of timeduring which high-temperature control is exercised decreases with anincrease in the cumulative lean time.

Process Performed by the Second Embodiment

FIG. 8 is a flowchart illustrating a routine that the microcomputer 30executes to implement the above functionality. The routine shown in FIG.8 first performs step 130 to determine whether the internal combustionengine is started up. If the determination result obtained in step 130does not indicate that the internal combustion engine is started up, thecurrently executed routine immediately terminates.

If, on the other hand, the determination result obtained in step 130indicates that the internal combustion engine is started up, the targettemperature for the sensor element 14 is set to an initial value (e.g.,550° C.). More specifically, the target impedance Ztg for use inimpedance feedback control is set to a value Z550 that corresponds to anormal target temperature (550° C.) (step 132).

Next, step 134 is performed to determine whether the water temperatureTHI prevailing at internal combustion engine startup is equal to orhigher than a predetermined determination temperature TH40 (e.g., 40°C.). If the determination result obtained in step 134 indicates thatTHI≧TH40, it can be concluded that the time interval between the lastinternal combustion engine stop and the internal combustion enginerestart is short, and that the amount of adsorbable species adsorptionis not enough to incur rich displacement on the sensor element 14. Insuch an instance, the current routine terminates while the targetimpedance Ztg remains to be Z550. After internal combustion enginestartup, impedance feedback control begins to be exercised over thesensor element 14. In this instance, therefore, power supply control isexercised over the heater 24 so that the temperature of the sensorelement 14 coincides with the normal target value (550° C.) hereinafter.

If, on the other hand, the determination result obtained in step 134does not indicate that THI≧TH40, it can be concluded that the degree ofadsorbable species adsorption to the sensor element 14 is unignorable.In such an instance, a high-temperature target value (e.g., 800° C.) isset for the sensor element 14 so as to exercise high-temperaturecontrol. More specifically, the target impedance Ztg for use inimpedance feedback control is set to a value Z800 that corresponds tothe high-temperature target value (800° C.) (step 136).

Although the high-temperature target value is set to 800° C. as anexample, an alternative high-temperature target value may also be used.The high-temperature target value should be higher than the normaltarget value so as to promote the desorption of the adsorbable species.For example, a high-temperature target value of approximately 700° C.will sufficiently promote desorption.

Next, step 138 is performed to determine whether the output of theoxygen sensor 10 is lean. The output characteristic of the oxygen sensor10 that prevails while the sensor element 14 is heated to a temperatureof approximately 800° C. does not significantly differ from the outputcharacteristic of the oxygen sensor 10 that prevails while the sensorelement 14 is maintained at a normal target temperature of approximately550° C. Therefore, even when high-temperature control is in progress,the output of the oxygen sensor 10 indicates with certain accuracywhether the exhaust air-fuel ratio is lean.

As described earlier, the adsorbable species adsorbed by the sensorelement 14 is urged to become desorbed in a lean atmosphere. Therefore,if it is found in step 138 above that the sensor output is lean, it canbe concluded that adsorbable species desorption is vigorous. In such aninstance, step 140 is first performed to increment a lean counter TAFLin order to reduce the duration of high-temperature control. Next, step142 is performed to correct a recovery determination value GAsumTG inaccordance with the count reached by the lean counter TAFL.

If, on the other hand, it is found in step 138 that the sensor output isnot lean, it can be concluded that adsorbable species desorption is notparticularly promoted. In this instance, the program flow skipsprocessing steps 140 and 142 so that the recovery determination valueGAsumTG prevailing during the preceding processing cycle is continuouslyused.

Next, the routine shown in FIG. 8 performs step 144 to calculate thecumulative value GAsum of the amount of air Ga that has been taken insince internal combustion engine startup. Step 146 is then performed todetermine whether the recovery determination value GAsumTG is exceededby the intake air amount Ga.

The microcomputer 30 stores an initial value for the recoverydetermination value GAsumTG. The initial value represents the cumulativevalue for the intake air amount GA that is required for the completionof adsorbable species desorption when high-temperature control isexercised over the sensor element 14 in a rich atmosphere. In step 142above, the recovery determination value GAsumTG is corrected by reducedthe length of time according to the counted value of the lean counterTAFL, that is, the length of time during which the exhaust pathatmosphere is made lean and the adsorbable species desorption ispromoted. As a result, the recovery determination value GAsumTGaccurately corresponds to the cumulative air intake amount GAsum that isactually required for the completion of adsorbable species desorption.

Therefore, if the determination result obtained in step 146 does notindicate that GAsum≧GAsumTG, it can be concluded that adsorbable speciesdesorption from the sensor element 14 is not completed. In such aninstance, the current routine terminates while the target impedance Ztgis maintained at Z800, and high-temperature control is continuouslyexercised over the sensor element 14.

If, on the other hand, the determination result obtained in step 146indicates that GAsum≧GAsumTG, it can be concluded that adsorbablespecies desorption from the sensor element 14 is completed. In thisinstance, the target temperature for impedance feedback control ischanged to a normal value (550° C.). More specifically, the targetimpedance Ztg is changed to Z550 (step 148). When processing step 148 isperformed, high-temperature control subsequently terminates and thennormal impedance feedback control starts.

When the process described above is performed, high-temperature controlis exercised after internal combustion engine startup so as to promoteadsorbable species desorption and reduce the period of time during whichthe sensor output suffers rich displacement. Further, the processensures that the completion of adsorbable species desorption preciselycoincides with the end of high-temperature control. As a result, thecontroller according to the present embodiment minimizes the influenceof oxygen sensor output deviation, which is caused by the adsorbablespecies, and begins to properly detect the exhaust gas statusimmediately after internal combustion engine startup without extra powerconsumption and without causing avoidable damage to the sensor element14.

Modifications of and Supplementary Information about the SecondEmbodiment

The second embodiment, which is described above, checks whether therecovery determination value GAsumTG is reached by the cumulative intakeair amount GAsum in order to determine whether adsorbable speciesdesorption is completed. However, an alternative determination methodmay be used. For example, the above determination may be formulated bychecking whether the duration of high-temperature control reaches to atarget time (recovery determination value).

Further, the second embodiment, which is described above, ensures thatthe longer the lean time, the smaller the recovery determination valueGAsumTG. In this manner, the second embodiment varies the duration ofhigh-temperature control in accordance with the phenomenon in whichadsorbable species desorption is promoted in a lean atmosphere. However,an alternative method may be used. More specifically, corrections may bemade to increase the cumulative intake air amount GAsum with an increasein the lean time so that the duration of high-temperature control variesaccording to the phenomenon.

In the second embodiment, which is described above, “recovery valuecounting unit” according to the fourth aspect of the present inventionis implemented when the microcomputer 30 performs processing step 144.Further, “heater control unit” according to the fourth aspect of thepresent invention is implemented when the microcomputer 30 performsprocessing steps 136 and 146. Furthermore, “cumulative lean timecounting unit” according to the fourth aspect of the present inventionis implemented when the microcomputer 30 performs processing steps 138and 140. Moreover, “determination value correction unit” according tothe fourth aspect of the present invention is implemented when themicrocomputer 30 performs processing step 142.

Third Embodiment Features of the Third Embodiment

The third embodiment of the present invention will now be described withreference to FIG. 9. The controller according to the third embodiment isimplemented when the hardware configuration shown in FIG. 1 or 2 isemployed to let the microcomputer 30 execute a routine shown in FIG. 9in place of or together with the routine shown in FIG. 5 or 7.

The controller according to the second embodiment exerciseshigh-temperature control over the sensor element 14 after internalcombustion engine startup, thereby reducing the period of time duringwhich the sensor output suffers rich displacement. The second embodimentconsiders the cumulative period of time after startup during which theair-fuel ratio is lean, and exercises high-temperature control untiladsorbable species desorption is completed.

The time required for adsorbable species desorption increases with anincrease in the amount of adsorbable species adsorption to the sensorelement 14 at the startup of the internal combustion engine. The amountof the adsorbable species increases with an increase in the length oftime during which the internal combustion engine is stopped. Therefore,the longer the internal combustion engine stop time, the longer the timerequired for adsorbable species desorption. The third embodiment variesthe duration of high-temperature control in accordance with the internalcombustion engine stop time in order to ensure that the duration ofhigh-temperature control precisely coincides with the time required foradsorbable species desorption.

Process Performed by the Third Embodiment

FIG. 9 is a flowchart illustrating a routine that the microcomputer 30performs to implement the above functionality. In the routine shown inFIG. 9, step 150 is first performed to determine whether the internalcombustion engine is started up. If the determination result does notindicate that the internal combustion engine is started up, step 152 isperformed to count the elapsed time TENGSP from an internal combustionengine stop. The present embodiment assumes that the elapsed time TENGSPcan be counted while the internal combustion engine is stopped.

If, on the other hand, the determination result obtained in step 150indicates that the internal combustion engine is started up, step 154 isperformed to set an initial value (e.g., 550° C.) as a targettemperature for the sensor element 14. Next, step 156 is performed todetermine whether a startup water temperature THI is equal to or higherthan a determination temperature TH40. If the determination of THI≧TH40is negative, the target temperature for the sensor element 14 is changedto a high-temperature target value (e.g., 800° C.) (step 158). Theseprocessing steps are the same as steps 132 through 136 in FIG. 8.

Next, the routine shown in FIG. 9 performs step 160 to determine whetherthe target time (hereinafter referred to as the “target retention timeT800TG”) during which the sensor element 14 should be maintained at ahigh-temperature target value of 800° C. is already calculated. If theobtained determination result indicates that the target retention timeT800TG is calculated, the program flow skips processing steps 162 and164, which will be described later.

If, on the other hand, the obtained determination result does notindicate that the target retention time T800TG is calculated, step 162is performed to calculate the target retention time T800TG in accordancewith the elapsed time TENGSP after an internal combustion engine stop.The target retention time T800TG (that is, the duration ofhigh-temperature control) should coincide with the time required foradsorbable species desorption. Therefore, it is required that the targetretention time T800TG increase with an increase in the amount ofadsorbable species adsorption, that is, with an increase in the elapsedtime TENGSP after an internal combustion engine stop. To meet the aboverequirement in the present embodiment, the microcomputer 30 stores a mapthat defines the relationship between the elapsed time TENGSP and thetarget retention time T800TG. In step 162, the map is referenced to setthe value T800TG that corresponds to the elapsed time TENGSP. As aresult, the longer the elapsed time TENGSP after an internal combustionengine stop, the longer the target retention time T800TG is set.

Next, the routine shown in FIG. 9 performs step 164 to clear the elapsedtime TENGSP after an internal combustion engine stop. In the subsequentprocessing cycles, the target retention time T800TG is maintained equalto the value calculated in the present processing cycle because thesteps 162 and 164 are skipped in those cycles.

After completion of the above processing steps, step 166 is performed tocount the length of time during which control is exercised to maintainthe sensor element 14 at a temperature of 800° C. (the time during whichZtg=Z800 or the time elapse after an element temperature of 800° C. isreached; hereinafter referred to as the “high-temperature control timeT800”). Next, step 168 is performed to determine whether thehigh-temperature control time T800 is equal to or longer than the targetretention time T800TG.

If the obtained determination result denies T800≧T800TG, it can beconcluded that adsorbable species desorption from the sensor element 14is not completed. In such an instance, the current routine terminateswhile the ensuing target impedance Ztg is maintained at Z800, andhigh-temperature control is continuously exercised over the sensorelement 14.

If, on the other hand, the determination result obtained in step 168indicates that T800≧T800TG, it can be concluded that adsorbable speciesdesorption from the sensor element 14 is completed. In this instance,the target temperature for impedance feedback control is changed to anormal value (550° C.) (step 170). As a result, high-temperature controlterminates and then normal impedance feedback control starts.

When the process described above is performed, high-temperature controlis exercised after internal combustion engine startup so as to promoteadsorbable species desorption and reduce the period of time during whichthe sensor output suffers rich displacement. Further, the process causesthe target retention time T800 to increase with an increase in theelapsed time TENGSP between an internal combustion engine stop and arestart, and ensures that the completion of adsorbable speciesdesorption precisely coincides with the end of high-temperature control.As a result, the controller according to the present embodimentminimizes the influence of oxygen sensor output deviation, which iscaused by the adsorbable species, and begins to properly detect theexhaust gas status immediately after internal combustion engine startupwithout extra power consumption and without causing avoidable damage tothe sensor element 14.

Modifications of and Supplementary Information about the ThirdEmbodiment

The third embodiment, which is described above, checks whether thetarget retention time T800TG is reached by the high-temperature controltime T800 in order to determine whether adsorbable species desorption iscompleted. However, an alternative determination method may be used. Forexample, the above determination may be formulated, as is the case withthe second embodiment, by checking whether the recovery determinationvalue GAsumTG is reached by the cumulative intake air amount GAsum (thevalue GAsumTG is set according to the value TENGSP in this instance).

Further, the third embodiment, which is described above, ensures thatthe longer the elapsed time after an internal combustion engine stop,the longer the target retention time T800TG. In this manner, the thirdembodiment varies the duration of high-temperature control in accordancewith the influence of the amount of adsorbable species adsorption.However, an alternative method may be used. More specifically, the rateof increase in the high-temperature control time T800 may be reducedwith an increase in the elapsed time TENGSP after an internal combustionengine stop so that the duration of high-temperature control is variedin accordance with the influence of the amount of adsorbable speciesadsorption.

Furthermore, the third embodiment, which is described above, does notvary the duration of high-temperature control in accordance with thelean time that arises after internal combustion engine startup. However,the present invention is not limited to such a scheme. Morespecifically, the controller according to the third embodiment may causethe duration of high-temperature control to decrease with an increase inthe lean time after internal combustion engine startup, as is the casewith the second embodiment.

In the third embodiment, which is described above, the high-temperaturecontrol time T800 corresponds to a “characteristics recovery value”according to the fifth aspect of the present invention, and the targetretention time T800TG corresponds to a “recovery determination value”according to the fifth aspect of the present invention. “Recovery valuecounting unit” according to the fifth aspect of the present invention isimplemented when the microcomputer 30 performs processing step 166.Further, “heater control unit” according to the fifth aspect of thepresent invention is implemented when the microcomputer 30 performsprocessing steps 158 and 168. “Stop time counting unit” according to thefifth aspect of the present invention is implemented when themicrocomputer 30 performs processing step 152. “Determination valuecorrection unit” according to the fifth aspect of the present inventionis implemented when the microcomputer 30 performs processing step 162.

Fourth Embodiment Features of the Fourth Embodiment

The fourth embodiment of the present invention will now be describedwith reference to FIG. 10. The controller according to the fourthembodiment is implemented when the hardware configuration shown in FIG.1 or 2 is employed to let the microcomputer 30 execute a routine shownin FIG. 10 in place of or together with the routine shown in FIG. 5 or7.

As described earlier, the desorption of the adsorbable species adsorbedby the sensor element 14 remarkably progresses in a lean atmosphere.Therefore, whether or not the desorption of the adsorbable species iscompleted can also be accurately determined by judging whether anadequate lean time is generated after internal combustion enginestartup. Therefore, the present embodiment checks the cumulative leantime generated after internal combustion engine startup to determine thetime at which high-temperature control over the sensor element 14 shouldterminate.

Process Performed by the Fourth Embodiment

FIG. 10 is a flowchart illustrating a routine that the microcomputer 30performs to implement the above functionality. In the routine shown inFIG. 10, processing steps 180 through 190 are the same as processingsteps 130 through 140 in FIG. 8. Processing step 194 in FIG. 10 is alsothe same as processing step 148 in FIG. 8. As regards the steps in FIG.10 that are the same as the steps in FIG. 8, their description isomitted or abridged to avoid a redundant description.

When the necessity for high-temperature control is recognized afterinternal combustion engine startup (step 184), the routine shown in FIG.10 sets a high-temperature target value (800° C.) as the targettemperature for the sensor element 14 (step 186). Next, the cumulativevalue TAFL of the time during which the exhaust air-fuel ratio is leanis counted in accordance with the output of the oxygen sensor 10 (step190). The above process is also performed by the routine shown in FIG.8.

Next, the routine shown in FIG. 10 determines whether the countedcumulative value TAFL of the lean time is equal to or larger than therecovery determination value TAFLTG (step 192). The microcomputer 30stores the recovery determination value TAFLTG, which is to be comparedagainst the cumulative lean time TAFL. The lean time that is requiredfor the adsorbable species adsorbed by the sensor element 14 to becomethoroughly desorbed after internal combustion engine startup is set asthe recovery determination value TAFLTG.

Therefore, if the determination result obtained in step 192 does notindicate that TAFL≧TAFLTG, it can concluded that the completion ofadsorbable species desorption is not recognized. In this instance, thecurrent routine terminates while the ensuing target impedance Ztg ismaintained at Z800, and high-temperature control is continuouslyexercised over the sensor element 14.

If, on the other hand, the determination result obtained in step 192indicates that TAFL≧TAFLTG, it can be concluded that adsorbable speciesdesorption is completed. In this instance, the target temperature forimpedance feedback control is changed to a normal value (550° C.) (step194). As a result, high-temperature control terminates and then normalimpedance feedback control starts.

When the process described above is performed, high-temperature controlis exercised after internal combustion engine startup so as to promoteadsorbable species desorption and reduce the period of time during whichthe sensor output suffers rich displacement. Further, the processdetermines the time of high-temperature control termination by payingattention to the cumulative lean time. Therefore, performing arelatively simple process makes it possible to ensure that thecompletion of adsorbable species desorption precisely coincides with theend of high-temperature control. As a result, the controller accordingto the present embodiment minimizes the influence of oxygen sensoroutput deviation, which is caused by the adsorbable species, and beginsto properly detect the exhaust gas status immediately after internalcombustion engine startup without extra power consumption and withoutcausing avoidable damage to the sensor element 14.

Modifications of and Supplementary Information about the FourthEmbodiment

The fourth embodiment, which is described above, determines the time inwhich the high-temperature control is performed in accordance with thecumulative lean time TAFL only. Alternatively, however, the time for thehigh-temperature control may be varied in accordance, for instance, withthe cumulative intake air amount GAsum that has been counted sinceinternal combustion engine startup, the high-temperature control timeT800 for the sensor element 14, or the elapsed time TENGSP after aninternal combustion engine stop. More specifically, the fourthembodiment may decrease the recovery determination value TAFLTG, whichis considered to a fixed value, with an increase in the value GAsum,decrease the recovery determination value TAFLTG with an increase in thevalue T800, and increase the recovery determination value TAFLTG with anincrease in the value TENGSP. Alternatively, the fourth embodiment mayincrease the rate of the increase in the cumulative value TAFL, which isconsidered to be fixed, with an increase in the value GAsum, increasethe rate of the increase in the cumulative value TAFL with an increasein the value T800, and decrease the rate of the increase in thecumulative value TAFL with an increase in the value TENGSP.

In the fourth embodiment, which is described above, “cumulative leantime counting unit” according to the sixth aspect of the presentinvention is implemented when the microcomputer 30 performs processingsteps 188 and 190, and “heater control unit” according to the sixthaspect of the present invention is implemented when the microcomputer 30performs processing steps 186 and 192. Further, “recovery value countingunit” according to the seventh aspect of the present invention isimplemented when the microcomputer 30 counts the elapsed time afterinternal combustion engine startup or the cumulative intake air amount,and “determination value correction unit” according to the seventhaspect of the present invention is implemented when the microcomputer 30corrects the tendency of the increase in the recovery determinationvalue TAFLTG or cumulative value TAFL in accordance with the abovecount. Furthermore, “stop time counting unit” according to the eighthaspect of the present invention is implemented when the microcomputer 30counts the elapsed time after an internal combustion engine stop, and“determination value correction unit” according to the eighth aspect ofthe present invention is implemented when the microcomputer 30 correctsthe tendency of the increase in the recovery determination value TAFLTGor cumulative value TAFL in accordance with the above count.

Fifth Embodiment Features of the Fifth Embodiment

The fifth embodiment of the present invention will now be described withreference to FIGS. 11 and 12. The controller according to the fifthembodiment is implemented when the hardware configuration shown in FIG.1 or 2 is employed to let the microcomputer 30 execute a routine shownin FIG. 11 in place of or together with the routine shown in FIG. 5 or7.

As described earlier, the output of the oxygen sensor 10 sufferstemporary sensor output rich displacement immediately after internalcombustion engine startup due to the influence of the adsorbablespecies. It is difficult to completely avoid adsorbable speciesadsorption. Therefore, it is also difficult to avoid sensor output richdisplacement. There is a correlation between sensor output richdisplacement and adsorbable species desorption amount. Further, theamount of adsorbable species desorption greatly correlates with theamount of adsorbable species adsorption prevailing at the beginning ofdesorption, that is, the initial amount of adsorbable speciesadsorption, and the subsequent elapsed time.

Therefore, the amount of adsorbable species desorption can be roughlyestimated from the initial adsorption amount and the elapsed time afterthe beginning of desorption. When the amount of desorption is estimated,it is possible to estimate the amount of sensor output richdisplacement, which is caused by the adsorbable species. When the amountof sensor output rich displacement is estimated, it is possible tocorrect the output of the oxygen sensor 10. Thus, the status of theexhaust gas can be accurately detected even while rich displacement isencountered. Under these circumstances, the present embodimentcalculates the amount of rich displacement superposed over the sensoroutput in accordance with the amount of adsorbable species adsorbed atinternal combustion engine startup and the elapsed time after thebeginning of desorption, and corrects the output of the oxygen sensor 10by using the calculated displacement amount as a correction value afterinternal combustion engine startup.

Process Performed by the Fifth Embodiment

FIG. 11 is a flowchart illustrating a routine that the microcomputer 30performs to implement the above functionality. In the routine shown inFIG. 11, step 200 is first performed to determine whether the internalcombustion engine is started up. If the obtained determination resultdoes not indicate that the internal combustion engine is started up,step 202 is performed to count the elapsed time TENGSP after an internalcombustion engine stop. The present embodiment assumes that themicrocomputer 30 can count the elapsed time TENGSP while the internalcombustion engine is stopped.

If the determination result obtained in step 200 indicates that theinternal combustion engine is started up, step 204 is performed to readthe sensor output OXSAD of the oxygen sensor 10. Next, step 206 isperformed to determine whether the temperature of the sensor element 14is equal to or higher than 300° C., that is, whether the temperature forthe start of adsorbable species desorption is reached.

If the determination result obtained in step 206 indicates that theelement temperature is lower than 300° C., it can be concluded thatsensor output rich displacement, which is caused by the adsorbablespecies, is not encountered yet. In this instance, step 208 is performedto set a final correction value KOXSR to zero. Next, step 226 isfollowed to perform a process for calculating the final sensor output asdescribed later.

If, on the other hand, the determination result obtained in step 206indicates that the element temperature is equal to or higher than 300°C., it can be concluded that rich displacement, which is caused by theadsorbable species, is encountered. In this instance, step 210 isperformed to count the elapsed time after an element temperature of 300°C. or higher is reached, that is, the elapsed time after the start ofadsorbable species desorption, as the “300° C. retention time T300”.

Next, the routine shown in FIG. 11 performs step 212 to determinewhether a normal target temperature of 550° C. is reached by theelement. If the determination result indicates that the elementtemperature is equal to or higher than 550° C., step 214 is performed todetermine that the oxygen sensor 10 is active.

After completion of the above processing steps, step 216 is performed tocalculate a correction amount initial value KOXSRI in accordance withthe elapsed time TENGSP, which is calculated while the internalcombustion engine is stopped. The correction amount initial value KOXSRIis a value for correcting the amount of rich displacement that isencountered at the beginning of adsorbable species desorption. Thisvalue KOXSRI should increase with an increase in the amount of initialadsorption. Therefore, the value KOXSRI should increase with an increasein the elapsed time TENGSP after an internal combustion engine stop.

FIG. 12A shows an example of a map that the microcomputer 30 referencesto calculate the correction amount initial value KOXSRI in step 216.This map is created to provide consistency with the amount of initialadsorption so that the correction amount initial value KOXSRI increaseswith an increase in the after-stop elapsed time TENGSP. According toprocessing step 216, a value accurately consistent with the amount ofrich displacement encountered at the beginning of desorption can be setas the correction amount initial value KOXSRI.

Next, the routine shown in FIG. 11 performs step 218 to calculate acorrection amount adjustment value KOXSRM in accordance with the 300° C.retention time T300. The amount of sensor output rich displacementdecreases as adsorbable species desorption progresses. Therefore, theamount of rich displacement gradually decreases with an increase in the300° C. retention time T300. The correction amount adjustment valueKOXSRM, which is calculated in step 218, corresponds to theabove-mentioned change in the amount of rich displacement.

FIG. 12B shows an example of a map that the microcomputer 30 referencesto calculate the correction amount adjustment value KOXSRM in step 218.This map is created to provide consistency with a change in the amountof rich displacement so that the correction amount adjustment valueKOXSRM increases with an increase in the 300° C. retention time T300.According to processing step 218, therefore, a value accuratelyconsistent with the amount of rich displacement reduction encounteredafter the beginning of desorption can be set as the correction amountadjustment value KOXSRM.

When the correction amount initial value KOXSRI and correction amountadjustment value KOXSRM are calculated as described above, the finalcorrection value KOXSR is calculated by the following equation (step220):

KOXSR=KOXSRI−KOXSRM  (1)

FIG. 12C illustrates how the final correction value KOXSR, which iscalculated from Equation (1) above, changes with time. Since thecorrection amount adjustment value KOXSRM increases with an increase inthe 300° C. retention time, the final correction value KOXSR graduallydecreases with an increase in the 300° C. retention time with thecorrection amount initial value KOXSRI applied as the maximum value asindicated in FIG. 12C. The changes in the final correction value KOXSRprecisely correspond to the amount of sensor output rich displacement,which is caused by adsorbable species desorption.

Next, the routine shown in FIG. 11 performs a guard process to ensurethat the lower-limit value for the final correction value KOXSR is zero(0) (steps 222 and 224). The output OXSAD of the oxygen sensor 10 andthe final correction value KOXSR are then substituted into the followingequation to calculate a final oxygen sensor output OXSF (step 226):

OXSF=OXSAD−KOXSR  (2)

When the process described above is performed, the final correctionvalue KOXSR that is accurately consistent with the degree of richdisplacement can be calculated after adsorbable species desorptionbegins following to the internal combustion engine starts up. When thecalculated final correction value KOXSR is then used to correct thesensor output OXSAD, it is possible to calculate the final oxygen sensoroutput OXSF from which the influence of rich displacement is eliminated.As a result, the controller according to the present embodimentminimizes the influence of oxygen sensor output deviation, which iscaused by the adsorbable species, and begins to properly detect theexhaust gas status immediately after internal combustion engine startup.

The fifth embodiment, which is described above, assumes that the degreeof rich displacement decreases with an increase in the 300° C. retentiontime T300, and determines the correction amount adjustment value KOXSRMas a function of T300. However, an alternative method may be used todetermine the correction amount adjustment value KOXSRM. Morespecifically, the correction amount adjustment value KOXSRM may bedetermined as a function, for instance, of the cumulative intake airamount GA prevailing after an element temperature of 300° C. is exceededor the lean time prevailing after an element temperature of 300° C. isexceeded.

In the fifth embodiment, which is described above, the 300° C. retentiontime T300 corresponds to a “desorption progress value” according to theninth aspect of the present invention and the final correction valueKOXSR corresponds to a “sensor output correction value” according to theninth aspect of the present invention. Further, the fifth embodimentimplements “element temperature acquisition unit” according to the ninthaspect of the present invention when the microcomputer 30 detects thetemperature of the sensor element 14 for impedance feedback control,“desorption progress value counting unit” according to the ninth aspectof the present invention when the microcomputer 30 performs processingstep 210, “output correction unit” according to the ninth aspect of thepresent invention when the microcomputer 30 performs processing step226, and “correction value calculation unit” according to the ninthaspect of the present invention when the microcomputer 30 performsprocessing step 218 and 220. Furthermore, the fifth embodimentimplements “stop time counting unit” according to the tenth aspect ofthe present invention when the microcomputer 30 performs processing step202 and “initial value setup unit” according to the tenth aspect of thepresent invention when the microcomputer 30 performs processing step216.

1. An exhaust sensor control system for an exhaust sensor mounted in anexhaust path of an internal combustion engine, wherein said exhaustsensor includes a sensor element for generating an output in accordancewith the status of an exhaust gas and a heater for heating said sensorelement, the exhaust sensor control system comprising: cumulative leantime counting means for counting, after internal combustion enginestartup, the cumulative length of time during which the air-fuel ratiois lean; and heater control means for controlling said heater with arecovery target temperature, which is higher than a normal targettemperature, set as a target temperature for said sensor element untilsaid cumulative length of time reaches a recovery determination value.2. The exhaust sensor control system according to claim 1, furthercomprising: recovery value counting means for counting the elapsed timeor the cumulative intake air amount after internal combustion enginestartup as a characteristics recovery value; and determination valuecorrection means for increasing said cumulative length of time ordecreasing said recovery determination value with an increase in saidcharacteristics recovery value.
 3. The exhaust sensor control systemaccording to claim 1, further comprising: stop period counting means forcounting stop period during which the internal combustion engine isstopped; and determination value correction means for decreasing saidcumulative length of time or increasing said recovery determinationvalue with an increase in the stop period during which the internalcombustion engine is stopped.